Stefanie, I do use small contractions in the stretched muscles, like gathering a good tension and release.
I’ve used the same principles applied to active stretching ( see Kurz books and videos)
I as a strength coach would love to know WHY the hell am I doing something.
The first reason for doing something is: “it works”.
As a strength & conditioning coach, you might leave to the scientists to find the reason why something works (although is always better to give an answer to the spoiled soccer players).
To find if something works you have to experiment.
Of course, not everything is worth experimenting with, that is left to the coach judgement according to his experience and knowledge.
Although the rationale given so far has been quite weak, the reported anedoctal experiences made the long holds worth a try, IMO.
Isometric contraction characterizes increased muscle tension, to varying degrees depending upon the position being held and any external load, while muscle length remains unchanged or only marginally changed as the duration increases and any slight lengthening may occur over the course of the sustained effort.
The static stretch is held against relaxed muscle(s) with the only tension/resistance being generated as a result of golgi tendon and muscle spindle activation at extreme amplitudes. The goal of which is to lengthen/re-lengthen the tissues involved.
The most significant distinction being that isometric hold, by definition, prohibits muscle relaxation during the effort while the static stretch requires it.
Good explanation.
So, from sprinting 100s only if you are a 100 m sprinter, we are now discussing doing isos whatever your sport? Wow!
Welcome to the early 1900s. Someone call me when Asafa (or Drob) gets to 9.6 via isos.
I don’t mind the thoughts re: isos. It’s just interesting how things go from the “absolute” generalization to the “extreme” specificity and back again… I reckon, their combo would be awesome! 
I think I know what you mean. Because I not only use iso holds, I also do rebound reps and altitude drops, that create repetitive contractions during the various stretches.
It does make sense in a way… Looking at science, and how it almost always takes each scientific revolution one full generation to turn over the old world view.
Each “generation” is hooked up to their current imprints. The revolution is complete only when a second generation, not hooked to the old imprint, is able to compare the two models and decides rationally which one works best.
I’m not positive Duxx, but I think because of the occlusion, the ST fibers fatigue much faster than normal, thus recruiting the type IIs. RJ?
Thanks so how does that explain the significant increase in flexibility noted by those that use extended Iso’s? 
Or are other factors introduced (more stretching) accounting for that and being attributed to the Iso’s…Stef?
I’m sure many have already read about EQI’s in CT’s book but thought in light of this discussion they are worth mentioning
[i] Bulgarian Squat, Eccentric Quasi-Isometric (EQI)
Tony Schwartz drew attention to EQI’s in Christian Thibaudeau’s e-book, Theory and Application of Modern Strength and Power Methods. It’s a great way to relax into stretch, and you’ll even work up a bit of a sweat in the process. Moreover, as Tony details in his write-up, EQI’s offer several benefits over traditional static stretching because the muscle to be stretched is contracting, thus shifting the emphasis from the series elastic component (SEC) to the parallel elastic component (PEC) (2). These benefits are structural, metabolic,and psychological. Check out the e-book for Schwartz’s outstanding review.
The Bulgarian squat is an excellent EQI for loosening up the hip flexors, especially if you become proficient enough to do it with added weight. Basically, you’re attempting to hold an isometric muscle action for as long as possible. However, as you fatigue, this isometric action will become a slow eccentric action, thus stretching the muscle in the “contracted” state. The same guidelines go as with the actual Bulgarian squat: keep the chest high and scapulae retracted.[/i]
Yes, the load present in this type of training is very low, but not much load is required to cause occlusion within a given muscle group. This state of occlusion generates a hypoxic environment, and in a hypoxic environment, the ST fibers have no way to generate fuel. If they have no way to generate fuel, they cannot contract. This leaves only the FT fibers to do the job.
From what I can gather, the inability of the ST fibers to do their job leads to the body adapting to a more anaerobic setting. One where the anaerobic metabolism and FT fibers are preferred over the aerobic metabolism and the ST fibers.
So, Im not sure if it’s a matter of recruiting the FT fibers that causes the change, or if it’s the inability to recruit the ST fibers, or maybe it’s a combination of both.
Any increase in flexibility is attributed to an isometric exercise that is held in an extreme lengthened position. Remember, depending on which literature we reference, I have not reviewed any material that stated that isometric work carried over to more than a relatively small amount of degrees of amplitude with respect to the associated tissues. This is merely reflective of specificity.
As a result, any isometric exercise that leads to an improved amplitude of movement about the associated joint(s) can only be attributed to holds in extreme lengthened positions relative to that individual’s flexibility.
No stretching at all. Just Iso’s.
i think you would compare to more to a static PNF stretch…
ie, in a classic static stretch your not contracting the muscle unlike in a PNF. During a ISO the muscle contracts - although not as hard as a PNF
Interesting… Thanks rj.
No problem. 
How would long sets of continuous tension (no pause at the bottom or up) done for >20 reps fit into this ‘theory’ of blood occlusion? CT uses them to specificaly hypertrophy ST fiber in his BB routines.
Sprinterouge,
I’ll put in some isos for me and for some tennis players to see the ‘effects’ altought I don’t have measuring devices to quantify them…
As a side note, I would love to know WHY not HOW (scientists). ‘Why’ should answer training goals questions, and how should answer physiological ones Can you post ‘goals’ if isos you want to achieve?
The problems with these types of sets is that even if there is always tension on the muscle, that tension will be much lower over most of the ROM, and therefore there will be more bloodflow. More bloodflow destroys the hypoxic effect. This is why ISOs are always held at the least leverage-advantaged joint configuration.
EDIT: Also, part of the benefit of long duration ISOs may be that they are actually very slow eccentric contractions. Enoka has some work showing that submaximal eccentric contractions recruit fast fibers preferentially. This could be another reason why long ISOs are better than long regular work sets.
Sprinterouge,
I’ll put in some isos for me and for some tennis players to see the ‘effects’ altought I don’t have measuring devices to quantify them…
As a side note, I would love to know WHY not HOW (scientists). ‘Why’ should answer training goals questions, and how should answer physiological ones Can you post ‘goals’ if isos you want to achieve?
I could say that “how” refers to practical modalities and “why” to the theoretical basis; but I understand now that your “why” was referring to the actuual goals of the method.
What I suppose I could benefit from is as follows:
Increased flexibility (injury prevention) and mobility (speed performance carryover).
Muscle mass retention in my legs in a almost completely alactic training phase.
Lactic acid tolerance training.
Additional effects in the rehab process of my right adductor.
Eventual performance carryover of increased strength at close knee angles (block clearance and first steps).
Any neurological benefit I could not be aware of.